The present invention relates to a load of particles for machining liquids for electro-erosion machines constituted by a dispersion of this load of particles in a dielectric liquid.
It is known that the presence of particles in the machining liquid of electro-erosion machines promotes triggering the electro-erosive discharges between the electrode-tool and the electrode-workpiece and reduces the time of achieving it. Swiss patents 670,785 and 627,393 disclose the use of conductive particles, such as aluminum, copper, tin, silver, zinc and alloys of these metals, or else semiconductive particles such as graphite. These particles are added in the form of powder to the dielectric liquid. The improvements of machining performance due to these powders depends largely on several factors, among which the most important are:
The concentration of particles in the dielectric liquid which have an optimum value for each type of particle;
The geometry of the grains, namely their rugosity, their dimensions and their shapes.
The best results have been obtained, prior to the present invention, with particles of very dissymetric shape, for example lamellae or flakes, as described in Swiss patent 670,785.
Loads of known particles, however, have a considerable non-homogeneity of the granulometry of the particles whose dimensions vary greatly within a given type of load.
The machining performances of these loads thus vary greatly and do not permit controlling precisely the machining parameters of the electro-erosion machine, such as the time of triggering the electro-erosive discharges, etc. Moreover, as the granulometry is greatly variable, the number of particles in the gap can greatly vary, even more or less to obstruct this latter, in particular in electro-erosive finishing and semi-finishing machinings.
Moreover, it is known that known loads give rise to pitting and irregularities of the surface in certain portions of the electrode-workpiece. FIG. 3 shows machining an electrode-workpiece 6 with an electrode-tool 5 separating by a working space or gap G filled with a machining liquid 7 comprising a load of conventional particles contained in a dielectric liquid. The load could for example be that described in Swiss patent 670,785, constituted by lamellae or flakes of all dimensions. Under the influence of the electric field produced during discharges, the largest lamellae orient by a point effect preferentially about the edges and the ends of the electrode-tool in the vicinity of which the electric field is the strongest. The alignment of the lamellae in this field gives rise to undesirable pitting of the electrode-workpiece. This pitting is the more pronounced the longer are the lamellae that are used.
The invention has for its object to overcome these drawbacks and to permit machining with improved, constant and predeterminable performance, and this even during semi-finishing and finishing machining, whilst ensuring high machining precision. It is characterized to this end by the fact that said load is comprised of carbon microfibers having a predetermined range of lengths and having a given diameter.
These characteristics ensure a load of particles having a well-defined and controlled granulometry, comprising microfibers of constant size and shape. There is thus obtained a controlled number of particles in the gap. The triggering time is shortened, permitting improved and constant machining performance from one load batch to another, without variation in time due to supply. There can also be enjoyed a saving in load, because a similar effect is obtained with a lesser quantity of microfibers. Moreover, pitting and surface irregularities on the electrode-workpiece can be avoided because of the controlled granulometry of the microfibers which thus do not form masses of large particles at the point of the electrode-tool. Machining precision is thus substantially improved.
These characteristics also ensure loads particularly well suited to finishing or semi-finishing machining, permitting that a controlled number of particles occupy the interval between the electrode-tool and the electrode-workpiece.
Preferably, the carbon microfibers are sectioned microfibers. These characteristics ensure particularly high, constant and controlled machining performance and a very regular machining without undesirable pitting of the electrode-workpiece facing the edges, points and asperities of the electrode-tool.
Preferably, the carbon microfibers have a predetermined length selected between 1 and 100 micrometers, preferably between 5 and 20 micrometers, the diameter of the microfibers being comprised between 0.2 and 8 micrometers.
These microfibers have a constant or statistically controlled length, permitting obtaining a load with optimum and constant characteristics for a given machining. Carbon is particularly well adapted for producing sectioned microfibers.
These carbon microfibers are preferably fibers of polyacrylonitrile that have been pyrolized and sectioned. There is thus obtained a high constancy as to diameter of the microfibers because the original polyacrylonitrile fibers have a substantially constant diameter.
According to a modification, the microfibers are carbon microtubes having closed or open ends.
The invention also relates to a process for production of a load of particles for machining liquids for electro-erosion machines, and is characterized to this end in that there are prepared carbon fibers from polyacrylonitrile fibers by thermal pyrolysis treatment adapted to expel oxygen, hydrogen and nitrogen, the fibers being reduced in size to obtain carbon microfibers having a predetermined range of lengths.
These characteristics permit rational, precise and inexpensive production of loads of well-calibrated carbon microfibers.
A preferred embodiment of the process is characterized by the fact that said predetermined range of lengths of the microfibers is a function of the width of the machining gaps intended for electro-erosion, this range being comprised between {fraction (1/10)} and 1 time (10% and 100%) the intended machining gaps, preferably between {fraction (1/6 )} and xc2xd (16.6% and 50%) the intended machining gap, more preferably between xc2xc and xc2xd (25% and 50%) the intended machining gap.
It is thus possible to obtain microfibers of a length perfectly adapted to the width of the machining gap such that a given number of sectioned microfibers can occupy the interval between the electrode-tool and the electrode-workpiece. This contributes to controlling and optimizing the wear, the dimensions, the deformation, the speed and the surface condition of machining.
The invention also relates to the use of a load of particles for machining liquids by electro-erosion, constituted by a dispersion of this load in a dielectric liquid, and it is characterized to this end by the fact that there is selected for the machining liquid a load comprised of carbon microfibers whose range of points is a function of the intended machining gap between an electrode-tool and an electrode-workpiece, and by the fact that this range of lengths is comprised between {fraction (1/10)} and 1 time the intended machining gap, preferably between ⅙ and xc2xd the intended machining gap, and more preferably between xc2xc and xc2xd the intended machining gap.
The invention also relates to a machining liquid and is characterized to this end by the fact that the concentration of carbon microfibers in the machining liquid is comprised between 0.25 and 4 g/l, preferably between 1 and 2.5 g/l.